Low pressure synthesis of electrically conductive platinum cobalt oxide from a platinum halide and an oxide containing cobalt



United States Patent O 3,414,371 LOW PRESSURE SYNTHESIS OF ELECTRICALLYCONDUCTIVE PLATINUM COBALT XIDE FROM A PLATINUM HALIDE AND AN XIDECONTAINING COBALT Donald B. Rogers, Carrcroft Crest, Wilmington, Del.,as-

signor to E. I. du Pont de Nemours and Company, Wilmington, Del., acorporation of Delaware No Drawing. Filed Jan. 31, 1967, Ser. No.612,774

8 Claims. (Cl. 23-50) ABSTRACT OF THE DISCLOSURE Process for preparingan electrically conductive oxide of the formula 3 where x is 0.6-0.8, byreacting a platinum halide, par ticularly PtCl2, with an oxidic reagentcontaining cobalt such as a cobalt oxide, alkali metal or rare earthcobaltate at 600-850 C. and autogenous pressure.

Field of the invention This invention relates to platinum cobalt oxide,and more particularly to an improved method for its preparation.

Prior Art A new electrically conductive platinum cobalt oxide which canbe represented by the formula PtXCoyOg, where x and y are 0.8i0.l2 andthe sum of x and y is less than 1.70, and which has a crystal structurebased on the rhombohedral space group 13m with cell edge w=6.164i0.05 A.

Summary and detailed description It has now been found that theelectrically conductive platinum cobalt oxide of the crystal structuredelined above can be prepared by a process which does not require theapplication of high pressures and thus can be done in less expensiveequipment. The process of this invention involves the preparation ofplatinum cobalt oxide having the crystal structure defined above andhaving the formula where x is 0.6-0.8, by reaction of a platinum halide;e.g., PtClZ with an oxidic reagent containing cobalt such as the cobaltoxides, e.g., COO and C0304, and .the alkali metal and rare earthcobaltates, e.g., LiCoOz and LaCoO3, at a temperature `of 60G-850 C.under autogenous pressure in a sealed reaction vessel. Preferably, thereaction is caried out at 700-750 C. with the oxidic reagent being onecontaining trivalent cobalt. Optionally, the oxidic reagent can 4be amixed cobalt oxide or a mixture of cobalt oxides containing bothdivalent and trivalent cobalt. The divalent eobalt oxide COO iseffective in exchanging oxygen for the halide ions liberated in thereaction.

3,414,371 Patented Dec. 3, 1968 lCe (Horton (1%)00304 ooo where thevalues of x can vary from 0.6 to 0.8. Equivalent quantities of reagentscontaining trivalent cobalt are used to obtain the highest phase purityin this process. When reactant compositions for which the values of xare less than 0.6 or greater than 0.8 are employed, some of the desiredhexagonal platinum cobalt oxide is formed, but the reaction mixture alsocontains excess C0304, metallic platinum, or other phases.

Another embodiment of the process of this invention involves themetathesis of a platinum halide and a mixed alkali metal-cobalt oxidecontaining trivalent cobalt, e.g., LiCoO2. This reaction takes placeaccording to the following equation:

heat

(1+x)PtC12 2LiCoOz d sealed container (1-l-x)PtClz -i- (2 2X 3 sealedcontainer As in the previous embodiments, proportions of reactants thatcorrespond to values of x ranging from 0.6-0.8 give the desiredhexagonal cobalt oxide in highest purity.

In still another embodiment, the platinum halide can be reacted withcobaltous oxide, CoO, alone. This reaction takes place with formation ofhexagonal platinum cobalt oxide that is always contaminated with otherphases. The process of this invention is conveniently carried out byplacing a mixture of the particular reactants in the proportionsindicated above into an inert reaction vessel capable of withstandingthe reaction conditions to be used, e.g., a platinum tube or a silicatube. The air in the reaction tube is then removed by suitable means. Inthe case of the platinum reaction tube, the charged tube is crimpedalong its length to expel air and the ends are then welded shut. In thecase of a silica reaction vessel, the charged tube is evacuated toremove air and then sealed shut. The reaction tube is then heated to thedesired temperatur. of 60G-850 C., and preferably at 70D-750 C., byconventional means, e.g., an electric furnace, for periods of timeranging from 16 to 36 hours or more. After heating is completed, thereaction vessel is cooled, opened, and the crystalline reaction productis removed. The product is leached in water to remove CoClz, or otherbyproducts, and the leached product is then washed with acetone anddried in air.

The platinum halides that can be used in the above process includePtF-2, PtCl2, PtBr2 and Ptlz. The oxidic reagents containing cobalt thatcan be used in the process include C0304; mixed alkali metal-cobaltoxides (alternatively named alkali metal cobaltates) of the formulaMCoOZ, where M is an alkali metal, e.g., LiCoOz, NaCoO3, or KCo03; mixedrare earth-cobalt oxides (alternatively named rare earth cobaltates) ofthe formula MCo03, where M is a rare earth of atomic number 57-64,inclusive, eg., LaCo03, PrCoO3, NdCoO3, SmCoO3 and GdC0O3, andco-baltous oxide, COO. The commercialy available grades of thesereagents can be used in the process of this invention, but it ispreferable to use grades of highest purity.

The process is illustrated in further detail in the following examples.

Example 1 A mixture of 0.4524 g. of PtCl2, 0.2048 g. of C0304, and0.0637 g. of C (that has been mixed by hand with an agate mortar andpestle) is placed in a platinium tube 31/2" long and 3/; in diameter.These quantities are in the proportions required by the formula Phi-x00O4 when x is 0.6. The platinum tube is crimped along its length to expelair and is sealed by welding. After heating in an electric furnace for36 hours at 750 C., the tube is cooled and opened. A highly crystallineproduct is obtained that is gray in color with a metallic luster andthat possesses a plate-like habit. After leaching in water to removeCoCl3, the crystallites are 4washed in acetone and air-dried. The X-raydiffraction pattern of the pulverized product shows the material to havethe same rhombohedral 'structure as CuFe03. The crystals belong to thespace group Rm with cell edge (1:6.164 A. and the rhombohedral anglea=26 34 (based on a hexagonal cell, a=2.83l A., 0:17.83 A.). Densitymeasurements of the polycrystalline sample indicate a density of about11.2 g./cc.

Example 2 A mixture of 0.4523 g. of PtCl3, C0304, and 0.0637 g. of COO(equivalent quantities corresponding to PtLsCoLGO) is placed in a tubeof fused silica in diameter and about 4 long. The t-ube is evacuated andsealed Iunder vacuum. The sealed tube is heated overnight (about 16hours) at 700 C. in an electric furnace. After heating is completed, thetube is cooled and opened. The resulting reaction product appearsvisually to be the same as that obtained in Example l. After leachingwith water as in Example l, the product gives an X-ray diffractionpattern identical with that given by the product of Example l.

Example 3 The procedure described in Example 2 is repeated with thesingle exception that a sealed platinum tube is used as the reactionvessel. The product of this experiment possess the same nominalcomposition and phase purity as the product of Example 2. The product ofthis example is combined with that of Example 2 'to provide a compositesample amounting to about 3 g. Chemical analysis of the compositeproduct gives the following re-l sults (expressed as Weight percent):Pt, 66.64%; C0, 21.17%; 0, 12.10, 12.99 and 12.91%. These results arewithin probable analytical error of those calculated for the nominalcomposition Pt1 6Co1V6O4.

is reacted as described in Example 2. After leaching in: water to removeCoCl3, the product is subjected to X-ray diffraction analysis and thepattern obtained indicates hexagonal platinum cobalt oxide plus a smallamount of C0304,

4 Example 5 A mixture of 0.1909 g. of PtCl2, 0.0704 g. of C0303, and0.0319 g. of

C00 (I=0.8 in P1+XC02 2X is reacted under the conditions disclosed inExample 2. An X-ray diffraction pattern of the leached product indicateshexagonal platinum cobalt oxide with no excess platinum and no excessC0303.

p Example 6 A mixture of 0.7056 g. of PtClz and 0.6388 g. of

Co3O4(z=O in the formula Pti+xCo2 O4) is reacted under the conditionsdescribed in Example 1. The crystalline product obtained contains thedesired hexagonal, platinum cobalt oxide phase contaminated with C0304,as indicated by X-ray diffraction analysis.

Example 7 A mixture of 0.4624 g. of PtCl3, 0.1542 g. of C0303, and0.0823 g. of

C00 (which corresponds to x=0.9 in Pti+xCo2 O4) Example 8 A mixture of0.2335 g. of PtCl2 and 0.1039 g. of LCOO2 (corresponding to x=0.6 in theformula Pt1+xCo2 O-i) is sealed into an evacuated silica tube asdescribed in Example 2. The closed tube is then heated at 750 C. for 24.hours. After cooling, the tube is opened. The reaction product isleached with water, and there is obtained approximately 0.25 g. ofpolycrystalline hexagonal platinum cobalt oxide. The X-ray diffractionpattern exhibited by thisproduct is identical to those obtained on theprodu cts of the preceding examples. No other phases are present.

Example 9 A mixture of 0.2262 g. of PtCl3 and 0.2090 g. of LaCOOa(quantities corresponding to z=0.6 in the formua Pti+xCo2 700 C. asdescribed in Example 2. The reaction product is leached in diluteaqueous HC1, washed in acetone, and air-dried. X-ray diffractionanalysis of the resulting product indicates that the hexagonal platinumcobalt oxide is the prilmary phase present, but it is contaminated withLaOCl.'L

Example 10 A mixture of 0.4650 g. of PtCl3 and 0.2620 g. of C00 isheated in a sealed platinum tube under conditions similar to thosedescribed in Example 1. The reaction product contains hexagonal platinumcobalt oxide, as shown by X-ray diffraction, contaminated with Pt andC0303.

The platinium cobalt oxide prepared by the process of this invention isuseful as a component in ceramic electrical resistor compositions asdescribed in the aforementioned application Ser. No. 612,775 filedconcurrently by R. D. Shannon. The use of platinum cobalt oxide made bythe process of this invention in electrical resistor compositions andthe application of such compositions to a ceramic dielectric base areillustrated below.

Finely divided platinum Cobalt Oxide, prepared by the proceduredescribed in Example 1, is mixed lwith powdered (ball-milled) glass ofthe following composition: SiO2, 25% by weight; PbO, 65%; and B203, 10%;and liquid organic medium consisting of 90% of -terpineol and of lowviscosity ethyl cellulose in the proportions listed in the followingtable to form a paste with a solid-to-liquid ratio of 2:1. Thecompositions are screen printed on small alumina plates and the coatedplates are then red with a 45-minute tiring cycle at a 760 C. peaktemperature, to produce electrical resistors. The resistors aremechanically strong and exhibit the sheet resistivities and thermalcoeilicients of resistance (TCR) summarized in the following table:

TABLE-ELECTRICAL RESISTORS CONTAINING PLATINUM COBALT OXIDE Sinceobvious modifications and equivalents in the invention will be evidentto those skilled in the chemical arts, I propose to be bound solely bythe appended claims.

The embodiments of the invention in which an exclusive property orprivilege is claimed are dened as follows:

1. The process of preparing an electrically conductive platinum cobaltoxide which comprises reacting,

at a temperature in the range 600-850o C. and at autogenous pressure ina sealed reaction vessel,

a platinum halide` and at least one oxidic reagent of the groupconsisting of cobalt oxides, alkali metal cobaltates and rare earthmetal cobaltates.

2. The process of claim 1 wherein the temperature is in the range700-750 C.

3. The process of claim 1 wherein platinum chloride is used.

4. The process of claim 1 wherein the oxidic reagent contains trivalentcobalt.

5. The process of claim 1 wherein a mixture of PtClz, C0304 and CoO isreacted together.

6. The process of claim 1 wherein in a -mixture of PtClz and LiCoO2 isreacted together.

7. The process of claim 1 wherein a mixture of PtCl2 and LaCoO3 isreacted together.

8. The process of claim 1 wherein a mixture of PtClZ and CoO is reactedtogether.

No references cited.

OSCAR R. VERTIZ, Primary Examiner.

H. T. CARTER, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,414,371 December 3, 1968 Donald B. Rogers It is certified that errorappears in the above identified patent and that said Letters Patent arehereby corrected as shown below:

Column l, lines 16 and 17, and lines 57 to 59, column Z, lines 7 to 9,26 to 28, and 40 to 42, column 3, lines 20 to Z2, and 68 to 70, column4, lines 4 to 6, 14 to 16, 25 to 27, 35

to 37, and SO to S2, the expression should appear as shown below:

Column Z, lines 5 and 6,

(l-x) should read (l- 3 3 same Column 2 lines 36 and 37,

(Z-Zx) V should read (v2-ZX) Column 3, line 39, after "PtClZ" insert0.2047 g. of

Signed and sealed this 17th day of March 1970.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. WILLIAM E. SCHUYLER, JR. Attesting OfficerCommissioner of Patents

